Probiotics are microorganisms that when consumed confer a positive or beneficial effect on the host. Use of probiotics in the oral cavity as preventative therapies are just beginning to be explored for dental caries and periodontal disease. Porphyromonas gingivalis (Pg) is an important organism in the onset and progression of periodontal disease. Current treatment for Pg associated periodontitis is largely based on treatment of symptoms rather than preventative therapies. We have isolated several probiotic strains of Lactobacillus delbreuckii from dairy products that inhibit Pg-induced cytotoxicity and Pg growth in vitro. Furthermore, we have demonstrated that supernatants derived from these strains of L. delbreuckii inhibit Pg growth. Initial testing indicates that the inhibitory molecule appears to be a protein with a molecular weight between 30-100 kDa. We propose to isolate and identify the secreted factor from L. delbreuckii by anion/cation exchange chromatography and mass spectrometry. Using this data, we will identify the genomic location encoding the inhibitory factor. We will clone and purify a recombinant inhibitory factor to carry out further testing. Using a previously constructed transposon library in Pg, we will use massively parallel sequencing technology to quickly and quantitatively screen for genes in Pg that interact with the inhibitory supernatant and/or recombinant inhibitory protein to quickly identify potential mechanisms of action. Lastly, we will examine the susceptibility profile of clinical isolates of Pg and other oral bacteria to L. delbreuckii inhibitory supernatants and/or purified recombinant inhibitory protein in an effort to determine spectrum of action of the L. delbreuckii inhibitor against clinical Pg isolates. Completion of the aims outlined in this application may identify a novel interspecies inhibitory mechanism and demonstrate the feasibility of a clinical application for our L. delbreuckii probiotic strains and/or their secreted products.